Fix sampling rules and sample rate reporting in environment logger

[yuandesu]

What does this PR do?

• Check tracing.sampling.default_rate as fallback for sample_rate
• Resolve null sampling_rules in diagnostic logs when sampler is unavailable by check tracing.sampling.rules when no sampler is configured

Motivation:

The sample_rate and sampling_rules are null even we configured via env var or Datadog.configure.

• dd-trace-rb/docs/GettingStarted.md

  Line 2103 in 9266d0a

  | `tracing.sampling.default_rate` | `DD_TRACE_SAMPLE_RATE` | `Float` | `nil` | Sets the trace sampling rate between `0.0` (0%) and `1.0` (100%). |

• dd-trace-rb/docs/GettingStarted.md

  Line 2105 in 9266d0a

  | `tracing.sampling.rules` | `DD_TRACE_SAMPLING_RULES` | `String` | `nil` | Sets trace-level sampling rules, matching against the local root span. The format is a `String` with JSON, containing an Array of Objects. Each Object must have a float attribute `sample_rate` (between 0.0 and 1.0, inclusive), and optionally `name`, `service`, `resource`, and `tags` string attributes. `name`, `service`, `resource`, and `tags` control to which traces this sampling rule applies; if they are all absent, then this rule applies to all traces. Rules are evaluated in order of declaration in the array; only the first to match is applied. If none apply, then `tracing.sampling.default_rate` is applied. |

Before:

app-1  | I, [2025-07-01T08:14:36.550791 #1]  INFO -- datadog: [datadog] (/usr/local/bundle/gems/datadog-2.17.0/lib/datadog/core/diagnostics/environment_logger.rb:15:in 'Datadog::Core::Diagnostics::EnvironmentLogging#log_configuration!') DATADOG CONFIGURATION - TRACING - {"enabled":true,"agent_url":"http://agent:8126?timeout=30","analytics_enabled":false,"sample_rate":null,"sampling_rules":null,"integrations_loaded":null,"partial_flushing_enabled":false}

After:

app-1  | I, [2025-07-01T08:16:06.774422 #1]  INFO -- datadog: [datadog] (/usr/local/bundle/gems/datadog-2.17.0/lib/datadog/core/diagnostics/environment_logger.rb:15:in 'Datadog::Core::Diagnostics::EnvironmentLogging#log_configuration!') DATADOG CONFIGURATION - TRACING - {"enabled":true,"agent_url":"http://agent:8126?timeout=30","analytics_enabled":false,"sample_rate":0.33,"sampling_rules":"[{\"service\":\"my-ruby-app\",\"sample_rate\":0.22}]","integrations_loaded":null,"partial_flushing_enabled":false}

Change log entry

Yes. Fix sampling rules and sample rate reporting in environment logger.

Additional Notes:

How to test the change?

You can reproduce this issue with the following files.

app.rb

#!/usr/bin/env ruby

require 'datadog'

Datadog.configure do |c|
  c.service = 'my-ruby-app'
  c.tracing.sampling.default_rate = 0.33
  c.tracing.sampling.rules = [
    {
      service: "my-ruby-app",
      sample_rate: 0.22
    }
].to_json
end

def main
  Datadog::Tracing.trace('do_something') do |root_span|
    puts "Do Something"
  end
end

begin
  main
rescue => e
  puts "Error: #{e.message}"
  puts e.backtrace
  exit 1
end 

Gemfile

source 'https://rubygems.org'

gem 'datadog', '~> 2.17'

Dockerfile

FROM ruby:3
WORKDIR /app
COPY Gemfile Gemfile.lock .
RUN bundle install
COPY app.rb .
CMD ["ruby", "app.rb"]

docker-compose.yaml

services:
  agent:
    image: datadog/agent:7
    volumes:
      - /sys/fs/cgroup/:/host/sys/fs/cgroup:ro
      - /proc/:/host/proc/:ro
      - /var/run/docker.sock:/var/run/docker.sock:ro
    pid: host
    cgroup: host
    environment:
      - DD_API_KEY=xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
      - DD_APM_ENABLED=true
      - DD_APM_NON_LOCAL_TRAFFIC=true
  app:
    build:
      context: .
    volumes: 
     # To test updated code.
      - ./environment_logger.rb:/usr/local/bundle/gems/datadog-2.17.0/lib/datadog/tracing/diagnostics/environment_logger.rb
    depends_on:
      - agent
    environment:
      - DD_AGENT_HOST=agent
      - DD_TRACE_DEBUG=true

[github-actions]

Thank you for updating Change log entry section 👏

^{Visited at: 2025-07-03 02:14:55 UTC}

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 97.52%. Comparing base (fd3e808) to head (2fd1aaf).
Report is 3 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #4772      +/-   ##
==========================================
- Coverage   97.54%   97.52%   -0.02%     
==========================================
  Files        1484     1483       -1     
  Lines       88499    88548      +49     
  Branches     4588     4594       +6     
==========================================
+ Hits        86322    86356      +34     
- Misses       2177     2192      +15     

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[marcotc]

@yuandesu great work! Can you add a changelog entry to the PR description, like it's mentioned in #4772 (comment)?

[yuandesu]

@yuandesu great work! Can you add a changelog entry to the PR description, like it's mentioned in #4772 (comment)?

@marcotc Thank you for the review!
I filled in the change log entry and added test as well. Please have a look when you have time.

[pr-commenter]

Benchmarks

Benchmark execution time: 2025-07-04 04:40:32

Comparing candidate commit 2fd1aaf in PR branch yuandesu/fix_null_sampling_rules_environment_logger with baseline commit fd3e808 in branch master.

Found 1 performance improvements and 1 performance regressions! Performance is the same for 45 metrics, 2 unstable metrics.

scenario:profiling - intern_all 1000 repeated strings

• 🟥 throughput [-1955.080op/s; -1885.825op/s] or [-8.125%; -7.837%]

scenario:tracing - Tracing.log_correlation

• 🟩 throughput [+10067.425op/s; +10375.011op/s] or [+10.077%; +10.385%]

[yuandesu]

/merge

[dd-devflow]

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